1. Field of the Invention
The present invention relates to a spherical surface acoustic wave apparatus comprising: a surface acoustic wave propagation substrate which includes, on an outer surface thereof, a surface acoustic wave circulation path configured to be annular and continuous using at least a part of a spherical shape, the circulation path being able to be excited to generate a surface acoustic wave therein and to allow the excited surface acoustic wave propagate and circulate therein in an annular and continuous direction thereof; a substrate supporting unit which is configured to support the surface acoustic wave propagation substrate; and a surface acoustic wave excitation/detection unit which is configured to excite the surface acoustic wave in the surface acoustic wave circulation path of the surface acoustic wave propagation substrate, to make the excited surface acoustic wave propagate and circulate in the annular and continuous direction, to detect the surface acoustic wave circulated in the surface acoustic wave circulation path, and to emit a reception signal corresponding to the detected surface acoustic wave.
2. Description of the Related Art
The above-described spherical surface acoustic wave apparatus is already well known, for example by FIGS. 1 and 7 of Japanese Patent Application KOKAI Publication No. 2005-94609.
The surface acoustic wave propagation substrate is formed by preparing a base material formed to include, on an outer surface thereof, a portion configured to be annular and continuous using at least a part of a spherical shape with a material which is not able to be excited to generate a surface acoustic wave, and by coating at least the annular portion on the outer surface of the base material with a material capable of being excited to generate the surface acoustic wave.
Alternatively, the surface acoustic wave propagation substrate is formed to include, on the outer surface thereof, the portion configured to be annular and continuous using at least the part of the spherical shape with a material capable of being excited to generate the surface acoustic wave.
A piezoelectric material is generally used as the material capable of being excited to generate the surface acoustic wave.
When forming an entire surface acoustic wave propagation substrate of the material capable of being excited to generate the surface acoustic wave, a piezoelectric crystal material such as quartz, lithium niobate (LiNbO3), lithium tantalate (LiTaO3), langasite (La3Ga5SiO14) and a family thereof is used. When forming the surface acoustic wave propagation substrate of the piezoelectric crystal material, the annular continuous portion is extended along a line at which a crystal face of the piezoelectric crystal material intersects with the outer surface of the piezoelectric crystal material. That is to say, in the outer surface of the piezoelectric crystal material, the surface acoustic wave can propagate only in an extending direction of the intersection line.
Then, the surface acoustic wave propagation substrate of the piezoelectric crystal material is formed into the spherical shape generally having a diameter of substantially 10 mm to substantially 1 mm in consideration of a manufacturing cost and various physical conditions required for circulating the surface acoustic wave (including a radius of curvature in a width direction of the annular continuous portion and a width and a frequency of the surface acoustic wave which should be excited in the annular continuous portion).
Although the surface acoustic wave excitation/detection unit may have various configurations, a so-called reed screen shaped electrode (also referred to as a comb-shaped electrode) is used in general in consideration of a manufacturing cost, an entire size of the unit, a conversion efficiency and the like.
The most basic reed screen shaped electrode has a shape in which a pair of comb-shaped terminal portions are combined such that a plurality of teeth like electrode branches of one comb-shaped terminal portions are alternately arranged with a plurality of teeth like electrode branches of another comb-shaped terminal portions.
The reed screen shaped electrode is directly formed on the annular continuous portion on the outer surface of the surface acoustic wave propagation substrate by for example photolithography (photoengraving), such that the teeth like electrode branches are arranged along an extending direction of the annular continuous portion. Alternatively, the reed screen shaped electrode is directly formed by for example the photolithography (photoengraving) on a bottom surface of a partial spherical concave of a reed screen shaped electrode supporting member independent of the surface acoustic wave propagation substrate, the partial spherical concave being formed so as to be similar to a part of the annular continuous portion on the outer surface of the piezoelectric crystal material. The reed screen shaped electrode formed on the bottom surface of the partial spherical concave of the reed screen shaped electrode supporting member is faced to the annular continuous portion on the outer surface of the surface acoustic wave propagation substrate through a predetermined gap (one-quarter or less of a wavelength of the surface acoustic wave to be excited) and is arranged such that the plurality of teeth like electrode branches are arranged along the extending direction of the annular continuous portion.
By applying in a burst manner a high-frequency signal of a predetermined frequency between the pair of comb-shaped terminals, it is possible to excite a surface acoustic wave having a wavelength corresponding to a period of arrangement of the plurality of the teeth like electrode branches of the pair of comb-shaped terminals combined to each other (that is to say, a distance between two teeth like electrode branches adjacent to each other) in the annular continuous portion on the outer surface of the surface acoustic wave propagation substrate, and a width of the excited surface acoustic wave corresponds to a length of each of portions of the two adjacent teeth like electrode branches, at the portions the two adjacent teeth like electrode branches being faced to each other.
The direction in which the plurality of the teeth like electrode branches of the pair of comb-shaped terminals of the reed screen shaped electrode are alternately arranged is substantially a direction of movement of a wave front of the surface acoustic wave excited as described above in the annular continuous portion on the outer surface of the piezoelectric crystal material. Therefore, the surface acoustic wave excited by the reed screen shaped electrode on the annular continuous portion on the outer surface of the surface acoustic wave propagation substrate propagates in the continuous direction of the annular continuous portion in the annular continuous portion.
For example, the International Publication No. WO 01/45255 A1 discloses an example of various condition settings for allowing the surface acoustic wave excited in the annular continuous portion on the outer surface of the surface acoustic wave propagation substrate to propagate in the continuous direction of the annular continuous portion and repeatedly circulate without diffusing in a direction orthogonal to the direction.
The condition includes a curvature of the annular continuous portion on the outer surface of the surface acoustic wave propagation substrate, a width of the surface acoustic wave (the length of each of the portions of the two adjacent teeth like electrode branches, at the portions the two adjacent teeth like electrode branches being faced to each other when the surface acoustic wave excitation/detection unit is the reed screen shaped electrode) excited in the annular continuous portion in the direction orthogonal to the continuous direction of the annular continuous portion (that is to say, the direction in which the excited surface acoustic wave is propagated), and the frequency of the surface acoustic wave excited in the annular continuous portion.
By providing a sensitive film sensitive to change in an external environment on the annular continuous portion (that is to say, the surface acoustic wave circulation path) on the outer surface of the surface acoustic wave propagation substrate, the spherical surface acoustic wave apparatus can measure a change in the external environment such as a gas concentration with which the sensitive film is brought into contact with high sensitivity.
In detail, the sensitive film changes a propagation speed of the surface acoustic wave which circulates in the circulation path and an attenuation rate of vibration energy of the circulated surface acoustic wave, corresponding to the change in the external environment such as the gas concentration with which the sensitive film is brought into contact, and further changes a time required for one circulation of a burst-like surface acoustic wave in the circulation path obtained by an output from the surface acoustic wave excitation/detection unit and changes a phase and strength of the surface acoustic wave for each circulation, so that the change in the external environment can be obtained by measuring these changes.
The phase in the high-frequency signal means a position of a predetermined high-frequency signal at a predetermined time in general. A phase measurement in the spherical surface acoustic wave apparatus is generally performed by measuring a position of a high-frequency signal output at a time after a predetermined time period has passed from a time at which the surface acoustic wave is excited using a Fourier analysis, quadrature detection or wavelet conversion based on the output generated from the surface acoustic wave excitation/detection unit at the time after the predetermined time period has passed. Then, from the phase obtained in this manner, the propagation (circulation) speed of the surface acoustic wave can be directly measured.
In this description, obtaining a passed time in the following manner is also referred to as “to measure the phase”. In this manner, for example, a passed time is obtained from a time at which the surface acoustic wave excitation/detection unit of the spherical surface acoustic wave apparatus applies the predetermined high-frequency signal in a burst manner to the annular continuous portion on the outer surface of the surface acoustic wave propagation substrate to excite the surface acoustic wave in the annular continuous portion, to a time at which the surface acoustic wave excitation/detection unit finishes to detect the surface acoustic wave which propagates and circulates on the annular continuous portion a predetermined number of times (that is to say, the time at which the surface acoustic wave finishes to circulate on the annular continuous portion a predetermined number of times), that is a passed time is obtained from the time of excitation to the time at which the circulations of the predetermined numbers is finished. The present invention does not exclude to obtain the propagation (circulation) speed of the surface acoustic wave in the annular continuous portion thereby.
For example, when a concentration of a predetermined gas in an external environment increases while the sensitive film of the annular continuous portion on the outer surface of the surface acoustic wave propagation substrate is in contact with the external environment and the sensitive film senses the concentration of the predetermined gas, a circulation speed of the burst-like surface acoustic wave which circulates in the annular continuous portion (that is to say, the surface acoustic wave circulation path) increases or decreases due to an effect of a change of the sensitive film which corresponds to the change in the gas concentration, and further a time required for one circulation of the burst-like surface acoustic wave in the circulation path decreases or increases. In this case, advance or delay occurs in a phase of the surface acoustic wave for each circulation and an attenuation rate of deterioration in strength increases or decreases.
Although each of the change in the circulation speed, the change in the circulation time, the advance or delay of the phase, and increase or decrease of the attenuation rate, each occurred by the above-described change in the external environment, is minute, each of the change, the advance or delay, and the increase or decrease is superposed with increase in the number of circulations of the surface acoustic wave in the circulation path and becomes larger. That is to say, accuracy in measurement of the change in the external environment is improved.
Therefore, when measuring the change in the external environment as described above using the spherical surface acoustic wave apparatus, it is clearly not preferable that the attenuation rate of the vibration energy of the surface acoustic wave which circulates in the circulation path increases and decreases by a factor other than the change in the external environment.
This invention is derived from the above described circumstances, and an object of the present invention is to provide a spherical surface acoustic wave apparatus which is simple in structure and easy to manufacture, which is capable of surely installing a surface acoustic wave propagation substrate including a portion annularly and continuously formed using a part of a spherical shape on an outer surface thereof to be able to excite a surface acoustic wave and to allow the surface acoustic wave to propagate and circulate therein, so that a change in an external environment can be measured always with high accuracy, and which is capable of changing the surface acoustic wave propagation substrate easily.